A closed ride height control system for vehicles of the type mentioned in the introduction is known from the European patent application EP 1 243 447 A2, which is incorporated by reference, and the German patent DE 102 31 251 C1, which is incorporated by reference. The closed ride height control system known from said documents has two or four controllable directional control valves, a pressure medium storage reservoir and at least two pressure medium chambers with in each case one controllable directional control valve. With said ride height control system, it is possible for pressure medium to be transferred from the pressure medium chambers into the pressure medium storage reservoir and vice versa. Furthermore, air can be transferred from the atmosphere into the pressure medium storage reservoir, with the compressed air being dried in an air dryer before entering into the pressure medium storage reservoir. To regenerate the air dryer, pressure medium from the pressure medium storage reservoir can be expanded via a throttle and conducted, in the opposite direction as compared with the air drying process, through the air dryer and via a discharge valve to the atmosphere. In the embodiment described in EP 1 243 447 A2, the air is sucked out of the atmosphere via a directional control valve, and is ventilated via the same directional control valve for the regeneration of the air dryer. In contrast, the embodiment according to DE 102 31 251 C1 has an additional air intake line which is connected to the compressor, such that only the ventilation takes place via the directional control valve and the abovementioned compressed air line.
Pneumatic ride height control systems require dry air in the system in order that the system components do not freeze and thereby become functionally impaired. For this purpose, a dew point is sought which prevents this.
In open ride height control systems and other pneumatic systems, in particular vehicle brake systems, the air delivered into the system minus a leakage can be used again for regenerating the dryer, such that virtually 100% of the delivered air is available for regeneration, and adequate dew points can be attained.
In both open and closed pneumatic systems, the regeneration can take place only when the compressor is not delivering compressed air into the system. Since compressed air consumption is higher in open systems than in closed systems, the compressor in open systems runs more frequently and for longer, and it is important to bring the regeneration cycles into line with times during which the compressor is not delivering compressed air.
For this purpose, DE 31 39 682 C2, which is incorporated by reference, discloses an air drying device for an open compressed air system, which has one or two humidity sensors arranged upstream and/or downstream of the air dryer and also has an air mass sensor. The regeneration of the air dryer is triggered by means of a pressure sensor every time a preset nominal pressure is reached in a compressed air storage reservoir. The regeneration time period is set as a function of the air humidity upstream and/or downstream of the air dryer.
EP 0 093 253 B2, which is incorporated by reference, discloses a control device for the regeneration of an air dryer for a pneumatic system, in particular of a vehicle brake system, in which, during idle operation of the compressor, in each case a regeneration air mass for drying is provided to the air dryer, which regeneration air mass is substantially proportional to the delivered air mass conducted through the air dryer during the preceding period of delivery operation of the compressor. If the time interval between two delivery cycles of the compressor is too short to allow the required regeneration air quantity to flow through the air dryer, measures are taken such that the regeneration air quantity deficit is added on to later regeneration cycles in order that the optimum drying of the air dryer can be made up during the next relatively long idle cycle of the compressor. Since the efficiency of the drying of the air dryer is lower at relatively low temperatures than at relatively high temperatures, a temperature sensor is provided which outputs a correction signal to the control device such that the regeneration air quantity determined in each case is dependent not only on the delivery rate of the compressor but also on the measured outside temperature.
In a further compressed air system described in EP 0 523 194 B1 (Published as DE 690 13 167 T2 and WO91/16224), which is incorporated by reference, a humidity sensor is provided which detects the humidity of the compressed air in the system and generates a regeneration control signal and a compressor control signal in order to interrupt the delivery of air by the compressor and regenerate the air dryer if the humidity of the compressed air in the system exceeds a predetermined value which indicates that the air dryer is saturated with moisture.
DE 196 20 851 A1, which is incorporated by reference, discloses an air treatment arrangement for compressed air, in particular for pneumatic brake systems of motor vehicles, which has a measurement device for the air quantity delivered through the air dryer. From the air volume thus determined, an electronic controller determines the point in time for the regeneration of the air dryer and the volume of the regeneration air required for the regeneration. A regeneration thus takes place only when a predetermined air quantity has flowed through the air dryer. The air quantity expended for the regeneration is dependent on the former air quantity. The regeneration air is extracted directly from the pressure system.
A method known from EP 1 173 270 B1, which is incorporated by reference, for regenerating air dryers is based on the volume of the dryer regeneration air required for regenerating the air dryer being determined in such a way that the system pressure, the outside temperature and the supplied air volume are measured and continuously transmitted to a control unit which, with regard to said parameters, controls the regeneration time and the supply of regeneration air to the air dryer.
With these systems and methods known from the prior art, it is sought to generate optimized regeneration cycles, which however cannot be directly transferred to closed pneumatic systems, in particular pneumatic ride-height control systems, because in closed systems delivered air may be discharged only proportionately, while on account of the way in which the system operates, compressed air must always remain in the system, and an adequate dew point must nevertheless be ensured.
Accordingly, the invention is based on the object of attaining, with as small a regeneration air quantity as possible, a dew point in the system which prevents freezing of system components and leaves a maximum air quantity in the system.